The acid dyestuffs were originally developed for the coloration of wool and consequently were provided with some degree of water solubility. It was generally believed that such dyestuffs went into solution before they affected coloration of the wool substrate. This mechanism was thought to be applicable to all of the acid dyes including those based upon anthraquinone chromophores. It was therefore believed that the crystal form of such dyestuffs was irrelevant to their dyeing behavior and was apparently not very intensively investigated.
Some of these dyestuffs were later found to have utility in coloring other types of fabrics such as polyamides but dissolving the dyestuffs remained an important feature of applying it to the substrate. The conditions of application and the nature of the substrate were changed but these changes provided no new motivation to investigate the crystal structure of the acid dyestuffs.
This lack of interest in the crystal structure is probably attributable to the fact that the acid dyestuffs are typically dissolved before application thus destroying any crystal structure which they might have possessed. This is in sharp contrast to the other great class of colorants, pigments, which are applied without dissolution. Therefore, both the crystal structures of pigments and methods of altering such crystal structures have been intensively studied.
The crystal modification of a dyestuff which does not dissolve in a dye bath but is believed to dissolve in the fiber being dyed has been reported. In particular, U.S. Pat. No. 4,388,078 reports the crystal modification of a disperse dye which alters its "pre-dyeing" property of aqueous dispersion stability.
The crystal form modification of textile dyeing auxiliaries which are dissolved before application have also been reported. In particular, U.S. Pat. Nos. 3,472,842, 3,511,833, 3,925,260 and 3,994,834 and United Kingdom Pat. Nos. 997,044 and 1,293,804 report the crystal form modification of optical brighteners in order to improve their color in dry formulations such as laundry detergents.